This invention relates to a method of forming fiber reinforced synthetic resin rod-like molding comprising a rod-like core having at least on the outer portion thereof a reinforcing fiber bundle impregnated with a thermosetting resin and a thermoplastic resin layer for coating the core, and particularly to a method for hardening the thermosetting resin during the process of manufacturing the rod-like molding.
A rod-like molding comprising a core portion having a FRP at least on the outer portion thereof and coated with a thermoplastic resin layer is utilized in various fields of industry because of its excellent physical properties such as tensile strength and modulus of elasticity. Heretofore, there has been a known method for manufacturing such fiber reinforced synthetic resin continuous rod-like molding, wherein a molten thermoplastic resin is supplied onto the outer periphery of the core portion formed of a reinforcing fiber bundle inpregnated with a nonhardened thermosetting resin. The coated core portion is immediately guided into a water basin so that only the thermoplastic resin layer is solidified by cooling while the thermosetting resin is kept nonhardened. Then this continuous rod-like semimolding with the nonhardened core is dipped into a hot water basin for hardening the thermosetting resin in order to obtain a desired tensile strength as well as modulus of elasticity.
Although the above mentioned method for treating the continuous rod-like molding by heating in the hot water basin provides a stable heating temperature and is very economical, when the thermosetting resin of the core portion and the thermoplastic resin of the layer are made of resins which do not have a good chemical affinity with each other, the intimacy or adhesion between them is not necessarily good. Such low adhesive strength decreases the mechanical strength of the rod-like molding and allows slippage of the thermoplastic outer layer relative to the core portion, which slippage is often caused by the fact that the thermosetting resin and the thermoplastic resin have difference coefficients of thermal expansion. In addition, this method limits productivity since a comparatively long time is required for the heat treatment.
The above method for manufacturing the fiber reinforced synthetic resin rod-like molding can be used, for example, to reinforce an optical fiber element. In that case, the optical fiber element is arranged in the center of the core portion, and enclosed by reinforcing fibers which are integrally adhered together by a thermosetting resin. In such optical fiber, the low degree of adhesion between the thermoplastic resin layer and the fiber reinforced resin of the core portion results in deterioration of the tensile strength, and flexural strength which could be one cause preventing this type of optical fiber from being put into practical use.
The present invention is provided in view of the above.